THE ROLE OF STORED HISTORIC SEDIMENT IN SHORT-TERM SEDIMENT PRODUCTION, SOUTH FORK NOYO RIVER, JACKSON STATE DEMONSTRATION FOREST, CALIFORNIA

In the South Fork Noyo River (SFNR) watershed, coastal northern California, large volumes of historic sediment were delivered to channels in response to past logging operations. This sediment presently is stored beneath historic terraces and in the present-day channels. Because current USEPA estimates of the Total Maximum Daily Load (TMDL) for the Noyo River were developed using office methods rather than field measurements, we conducted geomorphic mapping on the SFNR valley floor to quantify the volume of sediment stored in the watershed. We quantified sediment volumes associated with pre-historic terraces, historic terraces, and the active channel along four 1-mi-long stream reaches. Additionally, we established ten streamflow and suspended sediment sampling locations to monitor water and sediment discharges. We identified 158,000 cu. yds. of sediment stored in the active channel and 68,000 cu. yds. of sediment stored beneath historic terraces. These volumes are an order of magnitude less than the volumes calculated for pre-historic terraces. Based on analysis of surveyed cross sections we speculate that approximately 43 to 72% of the sediment originally stored beneath historic terraces has been transferred to the channel by erosion. The present-day channel sediment is stored presently in large gravel bars and is mobilized primarily during winter flood events.

Based on our channel mapping and hydrologic data, we infer that the largest suspended sediment loads are spatially coincident with the location of the greatest amount of stored channel sediment. Re-mobilized historic sediment appears to increase suspended sediment load, and may be a significant, unrecognized, sediment source. Thus, accurately mapping and quantifying channel deposits is a critical step for assessing sediment budgets, especially in TMDL studies attempting to relate upslope management to suspended sediment production.